Charge-transfer Chromophores Research Articles

The photophysics of photoredox catalysis: a roadmap for catalyst design.

Recently, the use of transition metal based chromophores as photo-induced single-electron transfer reagents in synthetic organic chemistry has opened up a wealth of possibilities for reinventing known reactions as well as creating new pathways to previously unattainable products. The workhorses for these efforts have been polypyridyl complexes of Ru(ii) and Ir(iii), compounds whose photophysics have been studied for decades within the inorganic community but never extensively applied to problems of interest to organic chemists. While the nexus of synthetic organic and physical-inorganic chemistries holds promise for tremendous new opportunities in both areas, a deeper appreciation of the underlying principles governing the excited-state reactivity of these charge-transfer chromophores is needed. In this Tutorial Review, we present a basic overview of the photophysics of this class of compounds with the goal of explaining the concepts, ground- and excited-state properties, as well as experimental protocols necessary to probe the kinetics and mechanisms of photo-induced electron and/or energy transfer processes.

Directed Assembly of Cuprous Oxide Nanocatalyst for CO2 Reduction Coupled to Heterobinuclear ZrOCoII Light Absorber in Mesoporous Silica

Hierarchical assembly of an oxo-bridged binuclear ZrOCoII light absorber unit coupled to a cuprous oxide nanocluster catalyst for CO2 reduction on mesoporous silica support is demonstrated. The proper positioning of the Cu oxide cluster was achieved by photodeposition of a [Cu(NCCH3)4]2+precursor by visible light excitation of the ZrOCo charge transfer chromophore, followed by mild calcination at 350 C. Illumination of the CuxOy-ZrOCo unit so formed in the presence of a diethylamine electron donor resulted in the reduction of surface Cu centers to Cu0 as demonstrated by the characteristic infrared band of adsorbed 13CO probe molecules at 2056 cm–1. For analogous CuxOy-TiOCoII units, the oxidation state makeup of the surface Cu centers was dominated by CuI, and the Cu0, CuI, and CuII composition was found to depend on the wavelength of MMCT excitation. The observed strong dependence of the CO2 photoreduction yield on the oxidation state of the surface Cu centers directly proves that CO2 is reduced on the C...

Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes

New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine dithiolate (PtN2S2) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H2 from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN2S2 CT chromophore. Each of the new dyads contains either a bipyridine (bpy) or phenanthroline (phen) diimine with an attached functional group that is used for binding directly to TiO2 nanoparticles, allowing rapid electron photoinjection into the semiconductor. The absorption spectra and cyclic voltammograms of the dyads show that the spectroscopic and electrochemical properties of the dyads are the sum of the individual chromophores (Bodipy and the PtN2S2 moieties), indicating little electronic coupling between them. Connection to TiO2 nanoparticles is carried out by sonication leading to in situ attachment to TiO2 without prior hydrolysis of the ester linking groups to acids. For H2 generation studies, the TiO2 particles are platinized (Pt-TiO2) so that the light absorber (the dyad), the electron conduit (TiO2), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H2O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO2 via a phosphonate or carboxylate attachment shows excellent light-driven H2 production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼ 40,000 turnover numbers of H2 over 12 d (with respect to dye).

Efficient Visible to Near-UV Photochemical Upconversion Sensitized by a Long Lifetime Cu(I) MLCT Complex.

The current investigation compares the photochemical upconversion sensitization properties of two long lifetime Cu(I) metal-to-ligand charge transfer (MLCT) chromophores to 3 distinct anthryl-based triplet acceptors. The sensitizers [Cu(dsbtmp)2](PF6) (1, dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) and [Cu(dsbp)2](PF6) (2, dsbp = 2,9-di(sec-butyl-1,10-phenanthroline) were selectively excited in the presence of anthracene, 9,10-diphenylanthracene (DPA), and 9,10-dimethylanthracene (DMA) in degassed dichloromethane solutions. In all instances, triplet energy transfer was observed from selective excitation of the Cu(I) MLCT chromophore to each respective anthryl species. The bimolecular triplet-triplet energy transfer quenching rate constants were extracted from dynamic Stern-Volmer analyses in each case, yielding values below the diffusion limit in dichloromethane. However, the Stern-Volmer quenching constants (KSV's) were sizable enough (up to ∼2300 M(-1) with 1 as a sensitizer) to support efficient photochemical upconversion. As such, visible to near-UV photochemical upconversion was observed in every instance, along with the anticipated quadratic-to-linear incident light power dependence when pumping at 488 nm. The latter verified that it is indeed sensitized triplet-triplet annihilation responsible for the generation of the anthryl-based singlet fluorescence. Photochemical upconversion quantum efficiencies were evaluated using a relative actinometric method as both a function of incident light power density as well as anthryl acceptor/annihilator concentration. When 1 was used as the sensitizer, upconversion quantum yields as large as 9.2% and 17.8% were observed for DMA and DPA, respectively. Finally, the combination of 1 with DMA was shown to be quite robust, showing no obvious signs of decomposition during 12 h of continuous 488 nm photolysis.

Ferrocenyl thiazoles: synthesis and properties

A set of ferrocenyl-substituted thiazoles 2 and 3 were designed and synthesized by Pd-catalyzed Sonogashira cross-coupling reaction. Their [2+2] cycloaddition–reteroelectrocyclization reaction with tetracyanoethene resulted in strong intramolecular charge transfer (ICT) chromophores 4 and 5. The electronic absorption spectra of 4 and 5 exhibit strong ICT bands in the near infrared (NIR) region. The electrochemical studies of ferrocenyl thiazoles 4 and 5 exhibit multistep redox waves. The computational studies show good agreement with the experimental results. The incorporation of 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) unit results is improved thermal stability of ferrocenyl thiazoles.

Altering molecular photophysics by merging organic and inorganic chromophores.

Photofunctional molecules and assemblies lie at the heart of many important fundamental processes in nature, and researchers have generated multitudes of artificial chromophores intended to mimic these naturally occurring systems. As dynamic spectroscopic techniques are becoming more widely available, ultrafast techniques in particular, substantial insight continues to be gleaned from the initial photon stimulation event through internal conversion, structural rearrangements, intersystem crossing, energy migration, electron transfer events, and ultimately regeneration of the ground state chromophores in both naturally occurring and inspired chromophores. This Account details research endeavors motivated by the concept that merging organic and inorganic chromophores can lead to new molecules exhibiting novel excited state properties. Moreover, these excited state properties can be fundamentally understood using combinations of static and dynamic spectroscopic tools, yielding systematic improvements to molecules poised for application in diverse research areas including light-harvesting, lifetime engineering, photocatalysis, and photochemical upconversion. Initial explorations focused on utilizing Förster energy transfer processes in Ru(II)-based metal-organic chromophores for solar light-harvesting while maintaining long excited state lifetimes. This eventually led to molecules exhibiting triplet-triplet energy transfer between energetically proximate triplet states featuring thermally activated photoluminescence from the upper charge transfer excited state with markedly extended lifetimes. Interest in systematically producing long-lived excited states with concomitant large Stokes shifts inspired the development of numerous Pt(II) bipyridyl and terpyridyl acetylide charge transfer chromophores featuring ultrafast intramolecular energy migration, high quantum yield ligand-localized phosphorescence at room temperature, and synthetically tunable excited state absorption properties. This structural motif also made it possible to access the triplet excited states of perylenediimide chromophores, permitting quantitative examination of internal conversion and intersystem crossing processes in these complex molecules. The generation of new metal-organic structures featuring unique photophysics appears limitless and simply requires the continued ingenuity of researchers.

Donor–acceptor–π–acceptor based charge transfer chromophore as electron donors for solution processed small molecule organic bulk heterojunction solar cells

Two benzothiazole (BT) based donor–acceptor–π–acceptor (D–A–π–A) molecular system denoted as BT3 and BT4 have been designed, synthesized and their optical and electrochemical properties were investigated. The BT4 show wider absorption profile and lower bandgap as compared to BT3 due to the strong electron withdrawing ability of dicyanoquinodimethane (DCNQ) as compared to tetracyanobutadiene (TCBD). The solution processed bulk heterojunction solar cells were fabricated using BT3 and BT4 as electron donor and PC71BM as electron acceptor. The organic solar cells optimized dichloromethane (DCM) processed BT3:PC71BM (1:1) and BT4:PC71BM (1:1) showed PCE of 2.56% and 3.68%, respectively. The higher PCE of BT4:PC71BM is related to the wider absorption of the blend and better ordered domain sizes in the blend as compared to BT3:PC71BM. The devices processed with 1,8-diiodoctane (DIO) additives showed PCE of 3.77% and 5.27%, for BT3:PC71BM and BT4:PC71BM blends, respectively.

Two-photon absorption properties of novel charge transfer molecules with divinyl sulfide/sulfone center

Organic materials with strong two-photon absorption response have attracted a great deal of interest in recent years for their many potential applications such as two-photon fluorescence microscopy, optical limiting, photodynamic therapy, and so on. Theoretical study on the relationships between molecular structure and two-photon absorption property has great importance in guiding the experimental design and synthesis of functional materials. Nowadays, quantum chemical calculations become very useful and popular tools in investigating the structure-property relations. At the same computational level, the two-photon absorption properties of different compounds can be compared accurately, and thus provide reasonable structure-property relations. Recently, a series of novel divinyl sulfides/sulfonesbased molecules have been synthesized and it is found that their photophysical properties behave like quadrupolar charge-transfer chromophores. In order to explore their potential two-photon absorption applications, in this paper, the two-photon absorption properties of these new molecules are calculated by using quantum chemical methods. Their molecular geometries are optimized at the hybrid B3LYP level with 6-31+g(d, p) basis set in the Gaussian 09 program. The two-photon absorption cross sections are calculated by response theory using the B3LYP functional with 6-31g(d) and 6-31+g(d) basis sets respectively in the Dalton program. In response theory, the single residue of the quadratic response function is used to identify the two-photon transition matrix element. Using the same methods, the two-photon absorption properties of distyrylbenzene compounds are computed for comparison. The basis set effects on excitation energies and two-photon absorption cross sections have been checked. It is found that the use of large basis sets could probably provide better numerical results, but the overall property trends would not change. Calculations show that the molecule with a triphenylamine group has the largest cross-section due to its strong donor groups. The divinyl sulfones-based dyes have larger cross-sections than the corresponding sulfides-based ones, because divinyl sulfones have stronger capability to accept electrons and at the same time the torsional angles between benzene rings in sulfones-based molecules are smaller than in the sulfides-based molecules. In the applicable wavelength range, these new dyes exhibit large two-photon absorption cross-sections which have the same order of magnitude as the strong two-photon absorption molecules with similar conjugation length. The largest cross section comes to 1613.3 GM calculated by using 6-31g(d) basis set. Molecular orbitals involved in the strongest two-photon absorption excitations are plotted and the charge transfer process is analyzed at length. The divinyl sulfide and sulfone centers behave as electron withdrawing groups and can form effective charge transfer molecules. On the basis of these new molecules, the structure inducing two-photon absorption enhancement is designed by employing isomerism effect. When the benzene rings of carbazole groups are connected with the molecular center, the planarity and charge transfer intensity are increased, and then the two-photon absorption cross-section can be improved dramatically. This study provides theoretical guidelines for the synthesis of new type of active two-photon absorption materials.

Vibrational coherences in charge-transfer dyes: a non-adiabatic picture.

Essential-state models efficiently describe linear and nonlinear spectral properties of different families of charge-transfer chromophores. Here, the essential-state machinery is applied to the calculation of the early-stage dynamics after ultrafast (coherent) excitation of polar and quadrupolar chromophores. The fully non-adiabatic treatment of coupled electronic and vibrational motion allows for a reliable description of the dynamics of these intriguing systems. In particular, the proposed approach is reliable even when the adiabatic and harmonic approximations do not apply, such as for quadrupolar dyes that show a multistable, broken-symmetry excited state. Our approach quite naturally leads to a clear picture for a dynamical Jahn-Teller effect in these systems. The recovery of symmetry due to dynamical effects is however disrupted in polar solvents where a static symmetry lowering is observed. More generally, thermal disorder in polar solvents is responsible for dephasing phenomena, damping the coherent oscillations with particularly important effects in the case of polar dyes.

Two States Are Not Enough: Quantitative Evaluation of the Valence‐Bond Intramolecular Charge‐Transfer Model and Its Use in Predicting Bond Length Alternation Effects

The structural weights of the canonical resonance contributors used in the Two-state valence-bond charge-transfer model, neutral (N, R1) and ionic (VB-CT, R2), to the ground states and excited states of a series of linear dipolar intramolecular charge-transfer chromophores containing a buta-1,3-dien-1,4-diyl bridge have been computed by using the block-localized wavefunction (BLW) method at the B3LYP/6-311+G(d) level to provide the first quantitative assessment of this simple model. Ground- and excited-state analysis reveals surprisingly low ground-state structural weights for the VB-CT resonance form using either this Two-state model or an expanded Ten-state model. The VB-CT state is found to be more prominent in the excited state. Individual resonance forms were structurally optimized to understand the origins of the bond length alternation (BLA) of the bridging unit. Using a Wheland energy-based weighting scheme, the weighted average of the optimized bond lengths with the Two-state model was unable to reproduce the BLA features with values 0.04 to 0.02 Å too large compared to the fully delocalized (FD) structure (BLW: ca. -0.13 to -0.07 Å, FD: ca. -0.09 to -0.05 Å). Instead, an expanded Ten-state model fit the BLA values of the FD structure to within only 0.001 Å of FD.

Donor-acceptor ligand-to-ligand charge-transfer coordination complexes of nickel(II).

A family of charge-transfer chromophores comprising square-planar nickel(II) complexes with one catecholate donor ligand and one α-diimine acceptor ligand is reported. The nine new chromophores were prepared using three different catecholate ligands and three different α-diimine ligands. Single-crystal X-ray diffraction studies on all members of the series confirm a catecholate donor-nickel(II)-α-diimine acceptor electronic structure. The coplanar arrangement of donor and acceptor ligands manifests an intense ligand-to-ligand charge-transfer (LL'CT) absorption band that can be tuned incrementally from 650 nm (1.9 eV) to 1370 nm (0.9 eV). Electrochemical studies of all nine complexes reveal rich redox chemistry with two one-electron reduction processes and two one-electron oxidation processes. For one dye, both the singly reduced anion and the singly oxidized cation were prepared, isolated, and characterized by EPR spectroscopy to confirm ligand-localization of the redox processes. The optical and electrochemical properties of these new complexes identify them as attractive candidates for charge-transfer photochemistry and solar-energy conversion applications.

Light induced carbon dioxide reduction by water at binuclear ZrOCo(II) unit coupled to Ir oxide nanocluster catalyst.

An all-inorganic polynuclear unit consisting of an oxo-bridged binuclear ZrOCo(II) group coupled to an iridium oxide nanocluster (IrO(x)) was assembled on an SBA-15 silica mesopore surface. A photodeposition method was developed that affords coupling of the IrO(x) water oxidation catalyst with the Co donor center. The approach consists of excitation of the ZrOCo(II) metal-to-metal charge-transfer (MMCT) chromophore with visible light in the presence of [Ir(acac)3] (acac: acetylacetonate) precursor followed by calcination under mild conditions, with each step monitored by optical and infrared spectroscopy. Illumination of the MMCT chromophore of the resulting ZrOCo(II)-IrO(x) units in the SBA-15 pores loaded with a mixture of (13)CO2 and H2O vapor resulted in the formation of (13)CO and O2 monitored by FT-IR and mass spectroscopy, respectively. Use of (18)O labeled water resulted in the formation of (18)O2 product. This is the first example of a closed photosynthetic cycle of carbon dioxide reduction by water using an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The observed activity implies successful competition of electron transfer between the IrO(x) catalyst cluster and the transient oxidized Co donor center with back electron transfer of the ZrOCo light absorber, and is further aided by the instant desorption of the CO and O2 product from the silica pores.

Dicyanobenzene and dicyanopyrazine derived X-shaped charge-transfer chromophores: comparative and structure-property relationship study.

A series of novel X-shaped push-pull compounds based on benzene-1,2-dicarbonitrile has been designed, synthesized and further investigated by X-ray analysis, electrochemistry, absorption and emission spectra, SHG experiment and quantum-chemical calculations. The obtained data were compared with those for isolobal 5,6-disubstituted pyrazine-2,3-dicarbonitriles. Structure-property relationships were elucidated. The extension, composition and planarization of the π-linker used as well as the electron-withdrawing ability of both dicyano-substituted acceptor units affect the linear and nonlinear properties of the target charge-transfer chromophores most significantly.

Protonation induced shifting of electron-accepting centers in intramolecular charge transfer chromophores: a theoretical study.

A new series of chameleonic molecules containing azulene and benzothiadiazole (BT) were designed and synthesized. In the neutral state, BT functions as an electron accepting center, while upon protonation, the electron accepting center shifts to azulene moieties, leading to a remarkable extension of absorption to the NIR region, i.e. up to 2.5 μm. The interchange between donor and acceptor characters upon protonation was confirmed by UV-vis-NIR spectral studies and supported by DFT calculations. Furthermore, the HOMO-LUMO level of ICT chromophores could be finely tailored by a different arrangement of azulenes and BTs in the molecules. The interchange between donor and acceptor characters upon protonation provides an alternative yet effective approach to fine tune optical and electronic properties of NIR chromophores.

Donor–Acceptor (D–A)‐Substituted Polyyne Chromophores: Modulation of Their Optoelectronic Properties by Varying the Length of the Acetylene Spacer

A series of donor-acceptor-substituted alkynes, 2 a-f, was synthesized in which the length of the π-conjugated polyyne spacer between the N,N-diisopropylanilino donor and the 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) acceptor was systematically changed. The effect of this structural change on the optoelectronic properties of the molecules and, ultimately, their third-order optical nonlinearity was comprehensively investigated. The branched N,N-diisopropyl groups on the anilino donor moieties combined with the nonplanar geometry of 2 a-f imparted exceptionally high solubility to these chromophores. This important property allowed for performing INADEQUATE NMR measurements without (13) C labeling, which, in turn, resulted in a complete assignment of the carbon skeleton in chromophores 2 a-f and the determination of the (13) C-(13) C coupling constants. This body of data provided unprecedented insight into characteristic (13) C chemical shift patterns in push-pull-substituted polyynes. Electrochemical and UV/Vis spectroscopic studies showed that the HOMO-LUMO energy gap decreases with increasing length of the polyyne spacer, while this effect levels off for spacers with more than four acetylene units. The third-order optical nonlinearity of this series of molecules was determined by measuring the rotational averages of the third-order polarizabilities (γrot ) by degenerate four-wave mixing (DFWM). These latter studies revealed high third-order optical nonlinearities for the new chromophores; most importantly, they provided fundamental insight into the effect of the conjugated spacer length in D-A polyynes, that can be exploited in the future design of suitable charge-transfer chromophores for applications in optoelectronic devices.

Cover Picture: Ferrocene-Donor and 4,5-Dicyanoimidazole-Acceptor Moieties in Charge-Transfer Chromophores with π Linkers Tailored for Second-Order Nonlinear Optics (Chem. Asian J. 2/2013)

Cover Picture: Ferrocene-Donor and 4,5-Dicyanoimidazole-Acceptor Moieties in Charge-Transfer Chromophores with π Linkers Tailored for Second-Order Nonlinear Optics (Chem. Asian J. 2/2013)

Solar Cell Efficiency, Self-Assembly, and Dipole–Dipole Interactions of Isomorphic Narrow-Band-Gap Molecules

We examine the correlations of the dipole moment and conformational stability to the self-assembly and solar cell performance within a series of isomorphic, solution-processable molecules. These charge-transfer chromophores are described by a D(1)-A-D-A-D(1) structure comprising electron-rich 2-hexylbithiophene and 3,3'-di-2-ethylhexylsilylene-2,2'-bithiophene moieties as the donor units D(1) and D, respectively. The building blocks 2,1,3-benzothiadiazole (BT) and [1,2,5]thiadiazolo[3,4-c]pyridine (PT) were used as the electron-deficient acceptor units A. Using a combination of UV-visible spectroscopy, field-effect transistors, solar cell devices, grazing incident wide-angle X-ray scattering, and transmission electron microscopy, three PT-containing compounds (1-3) with varying regiochemistry and symmetry, together with the BT-based compound 5,5'-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolobenzene}-3,3'-di-2-ethylhexylsilylene-2,2'-bithiophene (4), are compared and contrasted in solution, in thin films, and as blends with the electron acceptor [6,6]-phenyl-C(70)-butyric acid methyl ester. The molecules with symmetric orientations of the PT acceptor, 1 and 2, yield highly ordered blended thin films. The best films, processed with the solvent additive 1,8-diiodooctane, show donor "crystallite" length scales on the order of 15-35 nm and photovoltaic power conversion efficiencies (PCEs) of 7.0 and 5.6%, respectively. Compound 3, with an unsymmetrical orientation of PT heterocycles, shows subtle differences in the crystallization behavior and a best PCE of 3.2%. In contrast, blends of the BT-containing donor 4 are highly disordered and give PCEs below 0.2%. We speculate that the differences in self-assembly arise from the strong influence of the BT acceptor and its orientation on the net dipole moment and geometric description of the chromophore.

Thiazole azo dyes with lateral donor branch: Synthesis, structure and second order NLO properties

Abstract Three azo dyes with a thiazole based donor-π-acceptor structure containing a methoxyphenyl group as the lateral donor branch were synthesized and fully characterized. The formyl azo dye precursor was synthesized using 5-formylaminothiazole derivative as the diazo component and N,N-diethylaninline as the coupling component. Knoevenagel condensation of this precursor with different acceptors, namely, indandione, malononitrile and dicyanovinylindanone afforded the desired nonlinear optical chromophores a–c, respectively. The optical property, thermal stability and second order nonlinear optical activity were evaluated and a single crystal structural characterization was performed for two of the dyes. All nonlinear optical dyes a–c have good nonlinearity and good thermal stability. Compared with dyes a and b, dye c shows more than triple second order nonlinear optical response, the result that makes this molecule an unusual example of asymmetrical 2D charge transfer chromophore.

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